The present invention relates to a process for refining high impurity blister copper to anode quality. The process utilises alkali oxides and a solution containing sulphates to effectively remove sulphur and other impurities, such as As and Sb as well as Pb, Ni, Bi, Se and Te, from blister copper.
The production of blister copper from copper sulphide concentrates can be accomplished using two main pyrometallurgical systems: flash-smelting and bath-smelting. The number of stages within each system may vary from a single stage copper production to two stage smelting and converting processes.
A conventional two-stage smelting and batch converting process has the following major disadvantages: (i) the process is not energy efficient; (ii) the slag must be periodically skimmed from the converter; and (iii) the matte produced in the smelting furnace must be physically transferred to the converter furnace. During this transfer, high levels of fugitive emissions of SO2 are generated. Due to these drawbacks, there is a need to develop environmentally acceptable single stage smelting and converting systems that are both cost-efficient and energy-efficient.
Single stage blister copper production systems offers environmental and energy-efficiency advantages over the conventional two-stage copper smelting and batch converting processes. The Noranda continuous smelting and converting process is capable of producing blister copper from chalcopyrite concentrates in a single vessel. Likewise, the Noranda continuous converter is able to produce blister copper from mixtures of liquid and solid matte as well as from slag and copper concentrates. The Outokumpu flash smelting process can also produce blister copper from chalcocite concentrates in a single stage.
A significant drawback of all single-stage copper production systems is that they produce blister copper containing high levels of impurities, specifically sulphur, arsenic, antimony, and bismuth (e.g. ,1.3 wt % S, 0.5 wt % As, 0.5 wt % Sb, 0.03 wt % Bi). By comparison, blister copper produced in a conventional two-stage copper smelting and batch-converting process typically contains about 0.02-0.1 wt % sulphur, and only trace amounts of precious and other minor elements. However, as the grade and quality of copper ores decreases with time, even blister copper from conventional two-stage smelting-converting processes may contain high levels of these undesirable elements. Thus, in both cases, to produce molten xe2x80x9canode qualityxe2x80x9d copper, an extra xe2x80x9cblister copper refiningxe2x80x9d stage is needed.
Blister copper refining, which is the subject of this invention, is conventionally carried out in three steps: i) de-sulfurization; ii) fluxing-skimming; and iii) de-oxidation. In the first step, batches of molten blister copper are introduced into modified Pierce Smith converters or cylindrical xe2x80x9canode furnacesxe2x80x9d. Oxygen-enriched air is injected to remove the sulphur as SO2. As the sulphur content is lowered to about 0.003 wt % sulphur, the oxygen content reaches a level of about 0.8 wt %. Fluxing is practised by injecting basic materials such as mixtures of soda ash-CaO to combine with the acidic oxides of As and Sb, forming a slag that must be removed from the vessels prior to commencing de-oxidation. The oxidised molten copper thus produced is then de-oxidised to an oxygen level of about 0.1 wt % by injecting a reducing gas, such as natural gas.
Currently, various copper refining techniques exist for removing some of the impurities. The removal of arsenic by soda ash fluxing at about 1 wt % oxygen dissolved in copper is discussed by Eddy (1). The effectiveness of using soda ash fluxing to remove As and Sb is also described by Themelis (2). Stapurewicz et al. (3) provides a study of the thermodynamics of Sb removal from blister copper by soda ash fluxing. Taskinen (4) describes the distribution equilibrium of As, Sb and Bi between copper and soda ash. Peacey et al. (5) discusses the equilibria resulting from fluxing copper with soda ash and limestone. Riveros et al. (6) describes kinetic aspects observed during the operation and optimisation of the soda ash fluxing process practised at the Chuquicamata smelter.
In U.S. Pat. No. 3,561,952, the use of alkali oxide-silicate slags as well as alkali oxide phosphates and/or borates for the removal of lead and tin from copper scrap is described. In U.S. Pat. No. 3,262,773, a refining process for the removal of arsenic, antimony, tin and other acidic oxide forming elements from molten copper is presented. It teaches that removal may be accomplished by combining the acid oxides of such elements with basic materials such as alkaline earth oxides, in particular CaF2 and CaO, present in the slag. This patent suggests the use of 4 to 12 wt % calcium oxide based on the weight of the crude copper while the process temperature is maintained at between 1250 and 1300xc2x0 C. U.S. Pat. No. 4,316,742 describes a method of refining copper by melting the copper scrap in the presence of a flux that comprises a mixture of calcium oxide and sodium oxide in a weight ratio of CaO/Na2O of from 1:1 to 4:1 while bubbling oxygen into the copper bath. A process for the production of high-grade copper from an inexpensive starting material such as blister copper or copper scraps by adding a mixture of CaO and other oxides is disclosed in U.S. Pat. No. 4,055,415. U.S. Pat. No. 4,211,553 presents a method and apparatus for refining a melt using a pulverous solid material and a carrier gas, where the solid material may be CaO. U.S. Pat. No. 5,849,061 describes a stepwise injection of mixtures of air, oxygen and Na2CO3 followed by a simultaneous injection of hydrocarbons and SF6 as a process for refining high-impurity copper to anode quality copper.
While the above methods relating to the use of alkali oxides for the removal of impurities from molten copper have been described, they are not used in combination with a solution containing sulphates.
The concept of scrubbing SO2 and forming sulphates in pyrometallurgical systems is not new. For example, U.S. Pat. No. 4,034,036 describes a process for controlling SOx emissions from copper smelter operations involving pyrometallurgical reduction of copper ores to elemental copper, in which the gases from reverberating furnaces, roasters, and/or converters are scrubbed with a sodium alkali sorbent to produce sodium sulphate and sulphite wastes. This patent teaches the scrubbing of SO2 in flue gases. Conventional wet or dry scrubbing of SO2 in flue gases using sodium alkali, either using a regeneration type of scrubbing or a double alkali process, presents high capital and operating costs as well as environmental issues involving the disposal of a thixotropic sludge of the end product (i.e., calcium sulphate-sulphide).
A copper flash smelting process in which part of a sulfidic copper feed is roasted in the presence of a calcareous SO2 scavenger to produce a calcine containing calcium sulphate and an oxidic copper product is described in U.S. Pat. No. 4,615,729. This is referred to a sulphate roasting process where the sulfidic copper material is roasted at a temperature of about 850 to 1000xc2x0 C. The well-mixed feed therein is reacted with air to provide a calcine comprised mainly of solid calcium sulphate and copper ferrite and an off-gas rich in CO2 and poor in SO2. Thus, the concept of using an SO2 scavenger selected from the group of lime and limestone is established.
U.S. Pat. No. 5,180,422 describes a copper smelting process in which copper concentrates are smelted in a furnace to produce purified copper. The flue gases may be exhausted from either or both of a smelting furnace and a converting furnace, and gypsum may preferably be introduced into the converting furnace. In this process, the gas discharged from the furnace is treated to produce sulphuric acid. Gypsum is produced from the waste liquid treatment produced at the acid plant. Gypsum is recirculated to the converting furnace where it decomposes according to the reaction CaSO4=CaO+SO2+xc2xdO2. This approach is consistent with process slag chemistry since a source of lime is needed to produce a calcium ferrite slag, but the sulphate itself is not used to remove impurities from the melt.
Existing techniques for removing impurities from blister copper using mixtures of alkali oxides and/or carbonates have exhibited several disadvantages, including: i) excessive emissions of SO2 and volatile species like As and Pb during the sulphur removal and fluxing stages; ii) excessive refractory wear and unacceptable vessel mouth erosion that takes place during slag skimming; and iii) high cost of reagents. Therefore, there is a need to develop environmentally acceptable and cost-effective processes capable of effectively removing not only As and Sb, but also other impurities like Pb, Ni, and Bi from both low and high impurity blister copper. Accordingly, there is a great need for a simple process for refining blister copper, reducing to a minimum SO2 and volatile emission generation during the sulphur removal and fluxing stages, while shortening the process cycle. The most preferable result is a single stage sulphur and impurity removal process, wherein the SO2 and volatile components are removed in situ. Such a process would certainly be of great benefit to the industry, because the soda ash based system could be replaced with a sulphate based slag system, the latter producing significantly fluid slag at lower temperatures and lower refractory attack ability.
The contents of the following above-mentioned references are incorporated herein by reference: (1) C. T. Eddy, xe2x80x9cArsenic Elimination in the Reverberatory Refining of Native Copperxe2x80x9d, Transactions of the Metallurgical Society of the American Institute of Mining and Metallurgical Engineers, Vol. 96 (1931), pp. 104-118. (2) Themelis, N. J., xe2x80x9cInjection Refining of Directly-Smelted Copperxe2x80x9d, International Symposium on Injection in Process Metallurgy, TMS Minerals, Metals and Materials Society (1991), pp. 229-251. (3) Stapurewicz, T. T., and Themelis, N. J., xe2x80x9cRemoval of Antimony from Copper by Injection of Soda Ashxe2x80x9d, Metallurgical Transactions, Vol. 21B (1990), p. 967. (4) P. Taskinen, xe2x80x9cDistribution Equilibria of As, Bi, Cu, Pb and Sb between Molten Copper and Soda at 1200xc2x0 C.xe2x80x9d, Scandinavian Journal of Metallurgy, Vol. 11 (1982), pp. 150-154. (5) J. G. Peacey, G. R. Kubanek, and P. Tarrassoff, xe2x80x9cArsenic and Antimony Removal from Copper by Blowing and Fluxingxe2x80x9d, 109th AIME Annual Meeting, Las Vegas, Nev., February 1980. (6) Riveros, G. A., Salas, R. I., Zuniga, J. A., and Jimenez, O. H., xe2x80x9cArsenic Removal in Anode Refining by Flux Injectionxe2x80x9d, Mining in America, Institute of Mining and Metallurgy, Chatman and Hall, London, 1994. The contents of the following are also incorporated herein by reference: (7) T. Nakamura, Y. Ueda, F. Noguchi and J. M. Toguri, xe2x80x9cThe Removal of Group VB Elements (As, Sb, Bi) from Molten Copper Using a Na2CO3 Fluxxe2x80x9d, Canadian Metallurgical Quarterly, Vol. 23, No. 4, pp. 413-419, 1984.
The contents of the following list of U.S. Patents are incorporated herein by reference: U.S. Pat. Nos. 3,561,952; 3,262,773; 4,316,742; 4,055,415; 4,211,553; 5,849,061; 4,034,063; 4,615,729; 5,180,422; 5,516,498; 4,005,856; and 4,504,309.
In accordance with the present invention, there is now provided a novel process for refining high or low impurity blister copper using a solution containing molten sulphates. More specifically, the present invention comprises the steps of:
(a) Sulphur removal by injecting air/O2 gas mixtures into molten blister copper in the presence of an alkali source, over a period of time sufficient to complete the sulphur removal stage, with the innovation of removing the sulphur in situ, forming an effective amount of a molten alkali sulphate on top of the copper bath, the temperature in the vessel being maintained between 1100-1300xc2x0 C.
(b) Simultaneously injecting a solid alkali sulphate and/or basic oxide into the melt to promote the oxidation/fluxing (in situ scrubbing) of As and SB into a solution containing sulphates while the dissolved oxygen in copper increases from 0.1 to 0.6 wt %.
(c) Increasing further the level of oxygen in copper to about 1 wt % to further remove Sb, Pb, Ni and Bi by fluxing them into a molten solution of Cu2O and/or Cu2Oxe2x80x94CaO. The copper oxide and Cu2Oxe2x80x94CaO solution are stable with the sulphate phase and two immiscible liquid layers coexist in the vessel at the temperature of the process.
(d) Skimming the sulphate and oxide slag layers separately or together prior to commencing de-oxidation. The slags produced must be efficiently removed before reduction to avoid reversion of impurities back into the copper during the reduction phase.